The invention relates to a method of producing a screen having a structure of apertures in a black matrix and electroluminescent material in said apertures, on a display window of a color display tube, which method comprises the process steps of applying the black matrix and the electroluminescent material, in which process steps photosensitive material on the display window is exposed to light emitted by a light source and passed through a lens system and a shadow mask, which shadow mask is suspended from the display window and which lens system is positioned between the light source and the shadow mask, the lens system realizing, on the screen, a microscopic light distribution of the light originating from the light source radiating towards the screen.
The invention further relates to a color display tube and a display window provided with such a screen. Seminar Lecture Notes, Long Beach, Calif., May 15 and 19, 2000). This publication describes a method of applying the black matrix and electroluminescent material on the display window of a color display tube. This familiar way of producing a screen of a color display tube can be summarized by the following description of the major process steps.
First, the black matrix layer is applied. The display window is supplied with a photo resist layer, the mask is inserted and the layer is exposed in three consecutive steps so that all the areas thatxe2x80x94in a later process stepxe2x80x94will be filled by phosphors are exposed. After removing the mask, the locally hardened dots are developed with water and a layer of graphite is applied. The locally hardened dots are removed by an etching process, resulting in a graphite pattern that leaves open the areas where the phosphors will be applied.
In the second part of the screen producing process, the display window is provided with a photosensitive phosphor suspension layer. Then the shadow mask is inserted and the layer is exposed in such a way that only the areas on the display window that will be provided with a phosphor of a first color are illuminated, thus making the layer insoluble at the exposed locations. After this step, the layer is developed so that only phosphor remains at the proper locations. This process is repeated for the other phosphor colors.
In the present day exposure process, a light source radiates towards the display window and produces a microscopic light distribution behind the apertures of the shadow mask on the display window. The shape of this microscopic light distribution determines the robustness of the exposure process.
New trends in color display tubes, such as real flat tubes and slim tubesxe2x80x94i.e. color display tubes with a larger deflection angle, like for instance 120xc2x0 or morexe2x80x94make the exposure process much more difficult. Especially for real flat color display tubes provided with gun pitch modulationxe2x80x94as disclosed in EP-A-0968514xe2x80x94the exposure process will become more critical. A critical exposure process leads to lower yields in the production facilities and to color display tubes that show a decrease in picture performance, both by large spreads in the aperture size of the black matrix structure.
It is an object of the invention to overcome the disadvantage of the prior art method by providing a method of producing a screen with an improved response of the photosensitive system to the microscopic light distribution brought about by a more robust exposure process.
According to the invention, this object is achieved by means of a method which is characterized in that the photosensitive material comprises a bleaching dye functioning as a contrast enhancer for at least one of the said process steps.
The invention is based on the insight that the robustness of the exposure process can be significantly improved when the slope of the microscopic light distribution is steeper. This can be achieved by adding a bleaching dye to the photosensitive layer used in the exposure process for applying the black matrix or phosphors. The principle action of a bleaching dye in the exposure process for color display tubes will be described hereinbelow. In the prior art exposure process the microscopic light distribution, which determines the exposed area corresponding to an aperture in the shadow mask has a certain shape, i.e. a peak in the center and a circumferential area having a slope with a certain steepness. Now, when a photosensitive layer with a bleaching dye added to it is exposed to light, generating a microscopic light distribution as described, the bleaching dye will bleach, during the exposure process, as a result of which its transmittance will increase. The microscopic light distribution causes this bleaching process to occur relatively quickly in the center of the exposed area and more slowly towards the edges of the exposed area. Thus, in the course of the exposure process, the average transmittance of the photosensitive layer including the bleaching dye, is higher in the center of the exposed area than in the circumferential areas. This effectively results in a microscopic light distribution with increased steepness, that is with an enhanced contrast, which makes it possible to better define the process levels of the exposure process. This results in a more robust process and a better quality color display tube.
It should be noted that bleaching dyes are known per se; for instance, in U.S. Pat. No. 5,275,921 a bleaching dye is disclosed that is used in the production process of semiconductor elements. This process is totally different from the exposure process for color display tubes. In the pattern forming process of semiconductor elements, the mask used for exposing the photosensitive layer on the substrate is in close contact with the substrate. A problem in this process is formed by the reflections from the substrate. In U.S. Pat. No. 5,275,921, the bleaching dye is used for reducing the reflections from the substrate and to obtain a good contrast between the exposed and unexposed portions of the pattern. For that reason, the bleaching dyes in the semiconductor industry are applied as a separate layer on top of the photosensitive layer.
The use of a bleaching dye in the exposure process for color display tubes is based on its differential effect on the center portion and circumferential portion of the exposed areaxe2x80x94that is to say, the area on the display window exposed through an aperture in the shadow mask. This differential effect only occurs because the shape of the microscopic light distribution comes to a peak in the center and gradually slopes down towards the circumferential areas. So, this differential effect has to do with contrast enhancement within the exposed areas, not with improved contrast between exposed and unexposed areas. Preferably, the bleaching dye and the photosensitive material are applied in one process step, because this enables an introduction in the factories without major modifications to the production process. A two-layer system would require additional positions in the production line for applying and drying the bleaching dye. Despite this fact, a two-layer system should not be excluded as being one of the possibilities for contrast enhancement.
In a preferred embodiment, the bleaching dye is added to the photosensitive material for the process step in which the black matrix is applied.
In the manufacture of screens, the black matrix layer is applied first. The apertures in this black matrix structure determine the transmission of the matrix which is directly related to the luminance of the color display tube. The phosphor pattern is applied on top of the black matrix layer, the phosphor dots being somewhat larger than the apertures in the black matrix, in order to compensate for tolerances in the positioning of the phosphor pattern. For this reason, to obtain a high-quality screen a robust process for applying the black matrix is paramount.
A further embodiment is characterized in that the bleaching dye is soluble in water and forms a solution with the photosensitive material.
Most photosensitive materials used for the black matrix process are water-soluble. So, the production process is facilitated when the added bleaching dye forms a solution with the photosensitive material and is also soluble in water.
In a still further embodiment, the bleaching dye comprises a material of the group formed by 1,2-naphthoquinone-(2)-diazide-5-sulphonic acid sodium salt, 1,2-naphthoquinone-(2)-diazide-4-acid sodium salt, 4-diazodiphenylamine hydrogen-sulphate, 1-methyl-4-[2-(4-formylphenyl)ethenyl]pyridinium methosulphate. These four bleaching dyes show good characteristics for use in color display tubes, are water-soluble and are the materials that are preferably used.
A further embodiment is characterized in that the bleaching dye forms an emulsion with the photosensitive material. An alternative way of making a one-layer system consists in combining the photosensitive layer and the bleaching dye into one layer. The particles of the bleaching dye are not dissolved in photosensitive material, but form an emulsion.
In a still further embodiment, the bleaching dye coagulates after the emulsion has dried.
A bleaching dye of this kind has the advantage that, in the manufacturing process, the photosensitive layer and the bleaching dye are applied as a one-layer system, but during the drying process, the bleaching dye starts to coagulate, leading to a separation of the bleaching dye and the photosensitive layer, so that a two-layer system results.
A still further embodiment is characterized in that the time interval needed for the bleaching dye, when exposed to light, to increase its transmittance from 10% to 80% is between 5 and 30 seconds.
In the presently used processes for producing a screen for a color display tube, the time for exposing the photosensitive material is in the order of 10 to 30 seconds. In order to have a bleaching dye that has a differential effect on the central and circumferential areas of the microscopic light distribution, it is recommended to use a bleaching dye that discolorizes at the same rate as the rate needed for the exposure process.
The invention further relates to a color display tube and a display window provided with a screen which is produced using the method of the invention.